I. Field of the Invention
This invention relates to a method of enlarging and processing image data used for an enlarging copy machine which enlarges and prints the image data read from documents.
II. Description of the Background Art
Heretofore known is an enlarging copy machine in which the documents with a paper size of A4 or the like including a half tone image are read by an image sensor, thus read image data are data-processed and thereafter enlarged into a large size such as A1 or A0, and printed.
Conventionally, the image data in the enlarging copy machine of this kind are enlarged and processed as described below.
The image data before enlarging process including photographic density information are converted into pseudo half tone image data by pseudo half tone processing, where the pseudo half tone process ordinarily uses an error diffusion method, for example, the image data are converted into binary data so that the half tone density of 64 gradations is expressed by a density of dot pixels.
The pseudo half tone image data are enlarged, for example, by an enlarging and processing method disclosed in Japanese Patent Application Laid Open No. Hei-4-287568 in 1992. In the disclosure, the pseudo half tone image data are multiplied by an integer number, thereafter by calculating thus enlarged data, image data of a multi-valued density is produced. Such image data is then subjected to a thinning process, where the pseudo half tone image data are varied to an optional size. The image data of the varied multi-valued density are converted into binary digit data.
However in such conventional method of enlarging and processing, when enlarging by an integer multiplying number such as twice or three times, respective dot data in the image data are equivalently enlarged, therefore a quality variation does not arise. However in case of multiplying by a non-integer number such as 1.414 times, an image quality, in particular, a visual image quality is considerably deteriorated disadvantageously. One cause of such is considered that an outline portion of the image is made random (unclear) because a thinning process (or an insertion process of interpolation data) is performed for multiplying by the non-integer number, in addition to the pseudo half tone process using the error diffusion method and which is intrinsically a process in accordance with a periodicity.